Crude Oil and Natural Gas Properties and Equipment (Narrative) (Details) $ in Thousands, in Billions				12 Months Ended
	Sep. 26, 2018 USD ($)	Sep. 17, 2018 USD ($)	Sep. 17, 2018 XAF ( )	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 25, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Production license agreement term extended by government				5 years
Deferred tax assets, basis difference in fixed assets				$ 26,590	$ 38,479
Deferred tax liabilities, basis difference in fixed assets				18,600
Reduction in VAT receivable				275	(777)	$ (3,025)
VAT receivable, net of valuation allowance				4,200
Allocated to proved leasehold cost				22,500
Allocated to unproved leasehold cost				13,700
Estimated costs of technical studies				1,300
Estimated costs of technical studies, net				400
Undeveloped Acreage				$ 23,771	23,771
Monthly royalty rate, based on production at the published price				13.00%
Additional increase to working interest ownership, percent				0.80%
Inventory write-off				$ 0		0
Etame Field [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of participating interest				33.575%
Number of exploitation areas | item				3
Period of agreement for exploitation areas				10 years
Number of contract extension periods | item				2
Production license agreement term extended by government				5 years
Number of drilled wells required by agreement | item				2
Number of appraisal wells required by agreement | item				2
Block P Offshore Equatorial Guinea [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred tax assets, basis difference in fixed assets				$ 18,600
Working interest ownership, percentage				31.00%
Undeveloped Acreage				$ 10,000
Period of development				25 years
Capitalized costs, signing bonus				$ 6,700
Capitalized costs, tax impact				7,100
Undeveloped leasehold value				10,000
Gabon [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual commitment, gross							$ 65,000
Contractual obligation				1,200	$ 1,100	$ 1,200	$ 21,800
Cash paid for signing bonus, gross amount	$ 35,000
Payment of signing bonus, allocated to proved and unproved property	11,800
Reduction in VAT receivable, gross amount	25,000		14.1
Reduction in VAT receivable	$ 8,400	$ 4,200	4.7
Accrued liabilities, end of drilling activities, gross amount				5,000
Accrued liabilities, end of drilling activities				$ 1,700
Number of drilled wells required by agreement | item				2
Number of appraisal wells required by agreement | item				2
Working interest ownership, percentage					7.50%
Additional increase to working interest ownership, percent				2.50%
Inventory write-off					$ 400
Gabon DMO [Member] | Gabon [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual obligation				$ 1,100	1,200
PID And PIH [Member] | Gabon [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contractual obligation				$ 2,200	$ 1,900
Prior Production Sharing Contract, Through September 17, 2018 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Entitled percent for consortium after initial royalty percentage				70.00%
Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Entitled percent for consortium after initial royalty percentage				80.00%
Production Sharing Contract, After September 16, 2028 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Entitled percent for consortium after initial royalty percentage				70.00%